Method for producing oriented silicon-iron



3,271,203 METHOD FOR PRODUCING ORIENTED SILICON-IRON John L. Walter, Scotia, N.Y., assignor to General Electric Company, a corporation of New York No Drawing. Filed Oct. 16, 1962, Ser. No. 231,017 1 Claim. (Cl. 148--113) This invention relates to grain oriented magnetic alloys and more particularly to an improved method for producing bodies of iron base magnetic alloys having a majority of grains oriented in the (110) [001], or cube-onedge crystalline orientation which provides at least one direction of easiest magnetization.

Various procedures and processes have been used in the past to produce sheet and strip materials from siliconiron in which the strips or sheets have a cube-onedge orientation providing an easiest direction of magnetization parallel to the rolling direction. Generally, the basic concept behind the prior art processes has been to retain a small grain size during the processing rolling stages through the use of a dispersed second phase. This procedure, which is commonly used on materials of ordinary commercial grade, operates by including a dispersed second phase to prevent any appreciable grain growth from occurring during hot rolling or during annealing between cold rolling stages. It is not until the application of a final texture developing anneal that the second phase is either removed or dissolved and grain growth is effected. In the case of high purity silicon irons the situation is somewhat more difficult due to the fact that grain growth is not inhibited and final annealing temperatures much higher than those commonly used must be eiiected for significantly longer periods of time before the cubeon-edge orientation can be developed.

Although the (110) [001] or cube-on-edge crystalline orientation is well-known in electrical sheet steels and is referred to and discussed in many standard metallurgy texts, the fiollow-ing explanation of the orientation is included for the sake of clarity. The orientation may be described as one in which the unit cube lattices of the oriented grains have a plane containing diagonally opposite cube edges substantially parallel to the plane of the sheet and a pair of opposite cube faces substantially perpendicular to the rolling direction and to the plane of the sheet. The (110) [001] designation is based upon the Miller Crystallographic Index System, a complete discussion of which may be found in Structure of Metals, C. S. Barrett, 2nd edition, 1952, pages 125, published by the McMillan Company. Material having this orientation is anisotropic and has optimum magnetic proporties in the [001] direction parallel to the direction of rolling, the magnetic properties transverse to the direction of rolling being inferior to those of the rolling direction.

It is a principal object of this invention to provide an improved process for producing the (110) [001] grain orientation in iron base magnetic alloys of high purity.

Other objects and advantages of this invention will be in part obvious and in part explained by reference to the accompanying specification.

Generally, the present process involves providing a suitable starting strip prepared of an iron base magnetic alloy, containing not less than about 95 percent iron, balance substantially all silicon, and not more than about 0.2 weight per-cent incidental impurities. The starting strip may be produced by hot and cold rolling a starting blank according to proceses already generally wellknown and recognized in the art and can be of thicknesses ranging up to about mils, although the present process is particularly applicable for thinner gauges, that is, up to about 6 mils. Once the material of a predetermined ted States Patent "ice thickness has been provided, it is covered with a continuous coating of silica and then given an anneal in either vacuum or hydrogen for a time sufficient to develop the desired cube-on-edge orientation.

Considering a method by which grain oriented strip material can be produced according to the present invention, a starting ingot having a composition falling within the previously stated ranges, is heated to a temperature within the range of from about 700 C. to 1200 C. and hot rolled, with no intermediate anneals between hot rolling stages, to an intermediate or hot band thickness on the order of about mils (0.100 inch). At the hot band stage, the material is once again reheated to a temperature of from about 700 C. to 1200 C. and then cooled to room temperature or slightly above, for example, up to 200 C. to 300 C., and cold rolled to a thickness of up to about 15 mils (0.015 inch). If strip of lesser thickness is desired, then the 15 mil material can be reduced in cold rolling stages of not less than about 50 percent reduction each to whatever thickness is desired.

More specifically, the first step of the present process comprises providing a silicon-iron strip of predetermined thickness usually of 15 mils or less. This strip, which contains not more than about 0.2 weight percent total incidental impurities, can be produced by procedures wellknown in the silicon-iron technology. The strip can be hot rolled and cold rolled and given intermediate anneals between rolling stages according to existing practices as previously outlined. Suitable alloys are those which contain from 1.5 to 5.0 percent silicon and not more than about 0.2 weight percent total impurities.

Having provided a rolled strip of desired final thickness and preselected composition, the exposed surfaces of the strip are covered with a thin but continuous layer of silica. The thickness of the silica layer is not important to the invention as long as it is continuous, that is, free of breaks. The continuous layer of silica may be deposited on the exposed surfaces of the strip either by direct vapor deposition of silica or by subjecting the silicon-iron strip to an environment which will oxidize some of the silicon in the strip and form the required layer.

After the silica layer has been deposited, the coated strip is then annealed in an environment which will not remove the coating quickly. This annealing can be carried out in vacuum or hydrogen, the vacuum pressure being no higher than 10" millimeters of mercury and the hydrogen having a dew point no higher than about 40 F. The annealing is effected for a time sufficient to result in the proper cube-on-edge or [001] grain orientation being developed. As a practical matter, the time of the final anneal will generally run from one-quarter of an hour to ten hours depending on the thickness of the strip being treated.

As an example of the effectiveness of the present process in obtaining the cubeaon-edge orientation in high purity thin gauge silicon-iron strip more easily and more quickly than has heretofore been possible, samples of high purity (impurities 0.2%) 3 percent silicon-iron were cold rolled from the 13 to 15 mil thickness range to about 1 mil thickness in reduction stages of about 50 percent with intermediate anneals at 700 C. between rolling stages. All of the samples were cleaned and several then enclosed with a quantity of quartz in a container wherein the quartz was heated to about 1400 C. to vaporize the silica and deposit a continuous silica coating on the external surfaces of the strip. The deposition continued for from 15 to 30 minutes in all cases.

After deposition of the silica coating the strip samples were heated to a temperature of from 1000 C., temperatures of from 950 C. to 1100 C. being acceptable, and various samples held at this temperature for 1, 3 and 5 hours at evacuated pressures of 9 10'"' to 2 1O millimeters of mercury. Uncoated control samples were simultaneously processed in the same manner as those which had been coated to determine the effect of the coating on the formation of the cube-onedige orientation. Metallographic examination disclosed the fact that in all cases the coated samples had developed grain orientation 50 to 75 percent more complete than those of the cor-responding uncoated sample. For example, (the silica coated sample which had been heat treated for 5 hours was composed of about 90 percent cube-on-edge grains whereas the corresponding uncoated specimen consisted of less than about 50 percent secondarily recrystallized grains, this 50 percent being composed of a mixture of (110) and (100) oriented grains.

It is apparent that the present process for the first time makes it possible to obtain a strong cube-on-edge orientation by direct recrystallization in high purity thin gauge silicon-iron alloys.

What I claim as new and desire to secure by Letters Patent of the United States is:

In the method of producing cube-on-edge grain oriented strip the steps comprising, providing a rolled strip of final thickness, the strip compositionally consisting of from about 1.5 to 5.0 weight percent silicon, balance substantially all iron, and containing not more than about 0.2 weight percent incidental impurities and having no precipitated second phase to restrict grain growth, vapor depositing a continuous layer of silica over the exposed surfaces of the strip, and annealing the strip at a temperature of from about 950 C. to 1100 C. in an environment selected from the group consisting of vacuum no higher than about 10* mm. Hg and hydrogen having a dew point no higher than about F., the annealing being effected for a time sufiicient to orient a majority of the constituent grains in the [001] crystallographic orientation.

References Cited by the Examiner UNITED STATES PATENTS 2,385,332 9/1945 Heck 148-1 13 2,492,682 12/ 1949 Carpenter et al 148113 2,867,559 1/1959 May 148-111 2,939,810 6/1960 Fiedler et a l 148- 111 2,965,526 12/1960 Wiener 148--1 11 3,084,081 4/1963 Carpenter 148-113 3,105,781 10/1963 Walter 148113 3,106,496 10/1963 Anolick 148--112 3,125,473 3/1964 Schneider et al 148113 3,132,056 5/1964 McFuade 148-113 DAVID L. RECK, Primary Examiner.

W. B. NOLL, Examiner.

N. F. MARKVA, Assistant Examiner. 

